Heretofore, navigation apparatuses are known as apparatuses for route guidance in vehicles. These navigation devices basically function by continuously detecting a present position and displaying it on a map. They contain a GPS (global positioning system) apparatus for detecting the present position and a map database for storing map data. Furthermore, by entering a destination, the navigation apparatus searches for an optimum route from the present position and also guides the vehicle in a set travel route in the direction of movement at branch points.
Systems have also been proposed for connecting a mobile terminal, such as in a vehicle, with an information center, and obtaining information necessary in the vehicle. According to this type of system, restaurant information, sightseeing information, locations of usable parking lots, route guidance services to destinations, and so forth can be obtained as needed at the vehicle. The positions of and routes to restaurants, sightseeing spots, parking lots, and so forth can also be displayed on maps.
For example, a system is disclosed in Japanese Patent Laid-Open Publication No. Hei 7-72234 in which position information of a parking lot is returned as latitude and longitude data from an information center when information regarding the parking lot is requested from a terminal carried in a vehicle, and the position of the parking lot is displayed on a map contained in the terminal.
However, according to the apparatus of this publication, there is a problem where the displayed parking lot position based on the received latitude and longitude data is not a proper position on the map since the map contained in the terminal includes error. Namely, the origin material differs in creating the map or a certain amount of error is generated among a plurality of maps having different creation methods or precision. For this reason, a positional relationship with the peripheral condition (such as peripheral roads) of one point specified on one map may differ from a positional relationship with peripheral roads of the corresponding point on another map.
For example, even when a point specified by latitude and longitude is positioned on the right side of a road on one map, a situation may arise where it is indicated on the left side of the road on another map. Therefore, a situation may arise where the positional relationship with the peripheral roads may be incorrectly recognized at the vehicle, such as for a position of a parking lot specified by the information center.
To prevent this phenomenon, a method has been considered for specifying a point with respect to a map by a relative positional relationship with respect to roads (links). For example, this method (tentatively referred to as a relative positioning method) specifies a link by a link number, specifies the distance on the link from the starting point, and further specifies the direction and distance from the starting point. In this manner, by specifying the point as a relative positional relationship, breakdowns in the positional relationship of the road and the point to be specified can be prevented. Note that the links are organized in units of roads separated by intersections, and the road network is expressed as connections of these links in the map database.
The relative positioning method herein assumes that the link numbers at the vehicle and at the information center have identical definitions. However, due to differences in source materials and update times, there are instances where a link specified at the side executing a command does not exist. Furthermore, when there are differences in map precision, a link specified at the information center may not exist at the vehicle. In this instance, a problem is that it might not be possible to specify a point at the receiving side.